Currently, in many space missions, and notably those requiring the flight in formation of several satellites, the required absolute pointing accuracy is very high. To carry out the absolute pointing of observation instruments on board satellites, star sensors, also called stellar sensors, are currently used. These conventional star sensors have catalogues of stars whose absolute position is known with great accuracy. However, the star sensors make it possible to point towards a known bright star only approximately, with an accuracy of the order of several seconds of arc.
Thus, for absolute target systems of the state of the art, even the most recent, it is impossible to achieve the absolute pointing accuracies required for the missions currently envisaged, for which the required accuracies are of the order of a tenth of a second of arc.
Furthermore, it is particularly difficult to accurately calibrate this type of instrument: the mechanical and thermoelastic biases induced by the incorporation and launch, and the thermal environment, being virtually impossible to calibrate.
Consequently, the commonly-sought solutions consist:                either in developing new star sensors with the requisite accuracy, that is, less than a second of arc, but this involves heavy investments; furthermore, if such a solution did result in the development of a star sensor with the desired absolute accuracy, this would lead to a major extra cost and weight, which is not desirable, particularly for space applications,        or in seeking to accurately measure the induced biases, but this is very difficult and the residues are in any case generally estimated at several seconds of arc.        
One aim of the invention is notably to overcome the abovementioned drawbacks. Thus, to create an absolute target system provided with a maximum accuracy, the present invention proposes coupling a star sensor to an optical metrological sensor. Since these two items of equipment are normally already on board the satellites, in particular for formation flight missions, this solution adds no extra weight or cost.